Mechanics of breathing, gas distribution and functional residual capacity at different frequencies of respiration during spontaneous and artificial ventilation.

Nine healthy volunteers were investigated, both while awake and breathing spontaneously, and while anaesthetized with IPPV, in all cases at rates of both 12 and 24 b.p.m. Gas flow and volume were measured with a pneumotachography. The transpulmonary pressure (the pressure difference between the trachea or the buccal cavity and the oesophagus) was also recorded. The distribution of gas was analysed by means of nitrogen washout curves, which also permitted the determination of functional residual capacity (FRC). Lung compliance during IPPV was approximately half that during spontaneous breathing. During IPPV the compliance was dependent on the frequency of ventilation, being lower with the greater frequency. Pulmonary resistance was approximately twice as great with artificial ventilation, but no significant relationship to frequency was demonstrated. Gas distribution was within normal limits and in this respect there was no difference between low and high rates of spontaneous breathing. With IPPV at the higher rate, gas distribution was significantly less even, but still within normal limits. Dfferences in FRC under the different conditions during the experiments were not significant, but the values obtained were lower with artificial ventilation. Neither the reduction in dynamic lung compliance induced by anaesthesia and artificial ventilation, nor its dependence on the frequency of such ventilation, can be explained with certainty by changes in gas distribution.